ZINC DUST and the antimony precipitated with hydrogen sulphide and weighed as trisulphide or oxide. The silver corresponding with the amount of anti- mony found (Ag3Sb) is deducted from the total silver found, and the arsenic corresponding with the remainder calculated : 2As == i2Ag. * * # Refined zinc of good quality should not contain more than 1-5% of ex- traneous elements (Pb, Fe, Cd, Cu, etc.) and not more than 0-1% of As, Sb and S together. The presence of lead in small quantities (0-2-1-5%) in zinc is not very harmful, but with more than 2-3% of lead, zinc becomes brittle and diffi- cult to roll into sheets. Iron rarely -occurs in greater proportion than 0-2% and in such amount is not injurious, but in greater quantity it renders the metal hard and brittle. Cadmium also is always present in small proportion (o-oi 0-1%) and is not deleterious unless the zinc is to be used for making zinc white. Tin is found only in remelted zinc. Arsenic, antimony and sulphur are usually found in traces, but if together they exceed 0-1% the metal is very brittle ; a similar effect is exerted by copper to the extent of 0-5%. Carbon and silicon, which may sometimes be present in small proportions, have no appreciable injurious action. The compositions of various refined zincs of commerce are as follows (Schnabel, Nissenson, Hollard) : TABLE XXVIII Composition of Zincs Source. Zn1 Pb Fe Cd Ag Cu Sb ! As S Upper SUesia - j : ' 98-782 98-930 i-nS I-OOO 0-024 0-030 O-OI2 0-018 - - O'O22 ---- - Pulaski (U.S.) Bertha Spelter .... 99-981 0-019 ! _ . .,, I Font d'Art Vieille JVBM 98-904 1-070 0-016 o-oio 1 ----- Montagne [ estra _ 99-757 0-192 0-049 trace . O-O02 )I . . - 98-752 1-1524 0-0073 0-0705 O-OOO2 trace O-OO3O O-OOI5 0-0035 1 II . . . 99-868 0-1094 0-0065 0-0075 trace 0-0005 0-0018 0-0005 0-0005 0-0006 Commercial zinc dust constitutes a mixture of very finely divided zinc mixed with zinc oxide and variable proportions of cadmium, iron, lead, arsenic, carbon, etc., and is used as a reducing agent. It is usual to deter- mine its reducing power towards solutions of potassium dichromate or ferric salts; in some cases, however, the volume of hydrogen obtained under the action of dilute hydrochloric acid is measured, the quantity of zinc present and hence the commercial value being deduced therefrom. 1. Determination of the Reducing Power.Reagents: N/2-potas- sium dichromate (24-54 grams per litre) and N/2-thiosulphate.2 Procedure. I gram of the sample, 100 c.c. of the dichromate solution and 10 c.c. of dilute sulphuric acid (i : 3) are shaken for 5 minutes in a By difference. Fox the titration of thiosulphate solution, see Chrome Steels.recipitated as chloride, which is filtered off, washed, dried and weighed.